MariaDB support

[rafsaf]

I got feedback that support for MariaDB would be nice to have, I'm a little PostgreSQL biased, but MariaDB is awesome too.

• Replace postgresql dialect UUID field to backend agnostic UUID sqlalchemy field #38 first
• new option choice in cookiecutter for db
• some jinja ifs in docker-compose files and for sqlalchemy engine code
• maybe some other things will show up, not sure db session changes or if alembic configuration would survive it